1. Field of the Invention
The present invention relates to an electronic circuit device in which small surface-mount components are densely mounted on a circuit substrate using lead-free solder with deteriorated solder wettability and which is improved so as to prevent occurrence of soldering defects.
2. Description of the Related Art
In a solder connection between a connection terminal formed on a surface-mount component side and an electrode pad formed on a circuit substrate side, as the area of the electrode pad becomes smaller as the surface-mount component becomes smaller, it becomes more difficult for solder paste to be squeezed through openings in a metal mask onto the surface of the electrode pad, and a soldering defect may occur due to solder non-wetting phenomena on the electrode pad.
However, in the case of a small component in which the connection terminal is formed on aback surface of the surface-mount component to reduce the footprint thereof, it is difficult to detect the soldering defect.
With reference to FIG. 2 of Japanese Patent Application Laid-open No. 2002-353578 “SUBSTRATE FOR SURFACE-MOUNT COMPONENT AND METHOD OF MOUNTING THE SURFACE-MOUNT COMPONENT ON SUBSTRATE”, mounting position indication marks 14 indicating a normal mounting position A and misregistration indication marks 15 are formed at diagonal positions of a surface-mount component 1 mounted on a substrate 11. The surface-mount component 1 is adapted to be solder joined through molten solder balls 3 under a state of being mounted on the substrate 11 and being positioned with respect to one of the misregistration indication marks 15.
As a result, in the case of appropriate soldering, the surface-mount component 1 is normally moved to a position defined by the mounting position indication marks 14 by a self-alignment action. However, in the case of an unmolten solder material or insufficient wetting, the surface-mount component 1 does not move to the position defined by the mounting position indication marks 14 as the normal mounting position A, which can be visually detected.
Further, in FIG. 14 and paragraphs [0041] to [0043] of Japanese Patent Application Laid-open No. 2002-353578, there is disclosed that not the solder balls 3 (see FIG. 2) but the cream solder 16′ is applied to the surface-mount component 1, the misregistration indication marks 15 are displaced by substantially a half of a diameter L1 of a component-side electrode 2, and the component-side electrodes 2 and substrate-side electrodes 13 may be formed substantially in the shape of a circle or in the shape of a polygon such as a rectangle.
A “method of mounting a surface-mount component” in Japanese Patent Application Laid-open No. 2002-353578 can detect whether or not there are soldering defects at many electrode portions due to, for example, unsatisfactory temperature control in soldering in a large surface-mount component having a large number of electrodes, but is not appropriate for detecting a soldering defect with regard to one electrode among a large number of electrodes.
Further, even if a case is assumed in which the concept given here is applied to a surface-mount component having a small number of electrodes, when solder non-wetting in which solder is not applied to one connection surface occurs, the self-alignment effect acting on other connection surfaces can move the surface-mount component to an appropriate mounting position (because the inter-electrode pitch is the same between the component side and the substrate side), and thus, there is a problem that the state cannot be detected as a failure.
Further, the surface-mount component is intentionally mounted at an offset or displaced position, and thus, when the self-alignment effect does not act satisfactorily, a state occurs in which the item is determined to be non-defective but the surface-mount component is not mounted at an ideal position. There is a problem of a potential deterioration over time.